In general, modules for generation of a voltage reference represent one of the most important analog modules in the development of analog or digital circuits such as DRAMs, flash memories, voltage regulators, analog-to-digital converters, and other circuits.
The majority of voltage references are designed on the basis of a bandgap voltage reference that produces a reference voltage of approximately 1.25 V, said bandgap reference voltage having a low dependence upon the temperature and/or the supply voltage.
A bandgap voltage reference operates on the basis of the principle of balancing in a circuit the negative temperature coefficient of a pn junction, usually the voltage VBE on the base-emitter junction of a bipolar transistor, with the positive temperature coefficient of the thermal voltage VT, where VT=kT/q.
The characteristics of bipolar transistors enable them, as mentioned, to supply the best defined quantities in order to obtain positive and negative temperature coefficients. The thermal voltage VT has a positive temperature coefficient of 0.085 mV/° C. at room temperature; i.e., it is a coefficient of a PTAT (Proportional To Absolute Temperature) electrical quantity, whether voltage or current. Instead, the base-emitter voltage VBE of a bipolar transistor has a negative temperature coefficient of approximately −2.2 mV/° C. at room temperature; i.e., it is a coefficient of a CTAT (Complementary To Absolute Temperature) electrical quantity.
In general, a bandgap voltage reference adds together two quantities, a PTAT one and a CTAT one, in particular two voltages, so as to obtain a voltage reference with zero temperature coefficient. This is obtained, in particular, by multiplying a multiple M of the thermal voltage VT and adding it to the base-emitter voltage VBE, to obtain a reference voltage VREF=VBE+MVT.
In CMOS technologies, where independent bipolar transistors are not available, to obtain the PTAT and CTAT quantities indicated above, parasitic bipolar transistors are exploited, in a way in itself known.
It is also possible, to obtain PTAT voltages, to use the difference between the gate-source voltages of two weakly reverse-biased MOS transistors.
In what follows, reference will be made in any case to solutions for generation of a bandgap voltage reference that use the parasitic PNP bipolar substrate transistors available in CMOS technology.
FIG. 12 represents in this connection the structure of a pMOSFET M, obtained in CMOS technology, which shows how the regions with p+ doping of the MOS structure, the region with n doping of the n-well, and the p substrate together identify a PNP bipolar transistor. The references E, B, and C designate the emitter, base, and collector electrodes, respectively.
FIG. 1 shows an example of bandgap-voltage-reference generator, designated by the reference number 50, which uses parasitic PNP bipolar substrate transistors to generate a base-emitter voltage.
The above generator 50 basically comprises a circuit module 101 for generation of a base-emitter voltage difference, which comprises a pair of transistors, a first bipolar transistor Q1, and a second bipolar transistor Q2. These bipolar transistors Q1 and Q2 are obtained from the parasitic PNP bipolar transistors available in CMOS technology, as shown in FIG. 12. For this reason, the parasitic bipolar transistors Q1 and Q2 have the collector and the base connected to ground and hence connected in common. The second bipolar transistor Q2 has an aspect ratio that is a number N times that of the first bipolar transistor Q1.
The emitter terminals E1 and E2 of the bipolar transistors Q1 and Q2 define, respectively, two branches, B1 and B2, that correspond to the paths of the currents I from the supply voltage Vdd to ground GND through the two respective transistors Q1 and Q2 that provide the base-emitter voltage drop on the above respective branches.
Connected to the emitter terminal E1 on the first branch B1 is a first resistance R2, whereas connected on the second branch B2, between the emitter E2 and the supply voltage Vdd, are a second resistance R1 for adjustment of the bandgap reference voltage and a bias resistance R3. Connected to the emitter E1 of the first bipolar transistor Q1 and to the node between the adjustment resistance R1 and the bias resistance R3 are the positive and negative terminals of a differential amplifier AMP, which supplies at output the reference voltage VREF.
In this case, we have:VREF=VEB1+(R2/R1)VT·ln(N)where VEB1 is the voltage between the emitter and the base of the first bipolar transistor Q1. By operating on the ratio between the two adjustment resistances R2 and R1 and the value of the aspect ratio N, it is possible to vary the value of the bandgap reference voltage VREF.
FIG. 2 shows a circuit arrangement of a bandgap-voltage-reference generator 100, in which, as compared to the generator 50 of FIG. 1, the operational amplifier has been eliminated, introducing a third branch B3, with a third path from the supply Vdd to ground GND, through a third bipolar transistor Q3 set in parallel with respect to the transistors Q1 and Q2 that constitute the so-called bipolar core 101 of a voltage-reference generator 101.
In what follows, reference will be made to CMOS current mirrors, and the diode-connected MOSFET, which provides the current-voltage conversion, will be referred to as the first MOSFET or first transistor of the current mirror, and the other MOSFET connected thereto via the gate, which provides the voltage-current conversion, will be referred to as the second MOSFET or transistor of the current mirror.
In this case, the circuit includes a first CMOS current mirror 102 of an n type, which comprises a first MOSFET M1, which, as has been said, is diode-connected, with its gate and drain electrodes shorted, and a second MOSFET M2, and is connected between the first branch B1 and the second branch B2, and a second CMOS current mirror 103 of a p type, which comprises a first MOSFET M4 and a second MOSFET M3 and is connected between the first branch B1 and the second branch B2. The first and second current mirrors, 102 and 103, are complementary and connected, through nodes D1 and D2 corresponding to the drains in common of their MOSFETs so that each repeats current mirror the current of the other.
Present on the third branch B3 is a further MOSFET M5, connected to the gate of the first MOSFET M4 of the second current mirror 103, which provides a further current mirror in parallel to the second current mirror 103, the output of which is connected through a second adjustment resistance R2 to the emitter E3 of the third bipolar transistor Q3, thus completing the third branch B3. The voltage reference VREF is taken between the further biasing transistor M5 and the second adjustment resistance R2.
It should be noted that, together with the adjustment resistance R1 that connects the emitter E2 on the second branch to the source of the transistor M2 of the first current mirror 102, these current mirrors 102 and 103 provide substantially the structure of a ‘beta multiplier’, where, however, the MOSFETs M1, M2, M3, M4 all have the same aspect ratio so that the current I2 in the second branch B2 is equal to the current I1 in the first branch B1. Since also the MOSFET M5 has the same aspect ratio as the MOSFET M4, also the current I3 in the third branch B3 is the same.
Also in this case we obtain a relation similar to the previous one:VREF=VEB3+(R2/R1)VT·ln(N)where VEB1 is the voltage between the emitter and the base of the third bipolar transistor Q3, while R2 is the adjustment resistance connected to the emitter E3 of the third bipolar transistor Q3, and R1 is the adjustment resistance connected to the emitter E2 of the transistor Q2.
Hence, in general, known circuits use further power-consumption sources, and further operational amplifiers or bipolar transistors in addition to the pair of bipolar transistors that supplies the base-emitter voltage difference, thus preventing any reduction of consumption of the bandgap-voltage-reference generator.